The present invention generally relates to a method for improving synchronization between a satellite and a user terminal in a TDMA communications system. More particularly, the present invention relates to a method for minimizing the effect of co-channel interference between the TDMA synchronization bursts of a multi-beam communication system.
In a multi-beam processing satellite system, same frequency, same polarization signals may be used in multiple beams. Beams using the same frequency, same polarization signals are referred to as same xe2x80x9ccolorxe2x80x9d beams. In communications systems that do not employ polarization, beams sharing the same portion of the frequency spectrum are referred to as same color beams.
Considering two same color beams A and B, a portion of the signal from beam B may, at times, be observed in beam A as unwanted co-channel interference (CCI). For example, CCI may occur in beam B if beam A is operating at a very high transmit power in order, for instance, to overcome interference it is experiencing from users in other cells. CCI arises primarily because of the practical limitations in implementing an antenna system. Ideally, the antenna system would provide perfect rejection of same-color signals from other than the desired beam. However, in actual implementations, the coverage provided by beam A will inevitably provide some response to signals originating in beam B and vice-versa.
Many multi-beam satellite systems typically employ TDMA (time division multiple access) on the uplink to permit several user terminals to time share a frequency channel. In TDMA, a single frequency channel is typically shared among a number of users by assigning time slots to each user. Transmission bursts for each user occur within that user""s assigned time slot.
TDMA requires that all user terminals maintain precise synchronization so that their uplink bursts arrive at the satellite in a non-overlapping fashion. As part of the activity required to maintain synchronization, each user terminal may send, from time to time, an overt synchronization burst (SB) in a time slot and on a frequency channel dedicated to the user terminal. The satellite processes the SB and compares its time of arrival to its own timing so that the user terminal may be apprised of whether it""s timing is early or late relative to the satellites.
In previous systems, every same color beam used the same SB. Consequently, when an SB in a same color interfering beam (for example, beam B) aligned in time with the SB for a same color beam A, the signal from B coupled (interfered with) into the signal from A. Consequently, since the signal structure for both beams A and B was identical, the CCI from B was boosted to the same extent as the SB from A at the antenna for receiving A.
Consequently, any timing error in the user terminal transmitting beam B coupled into the processing electronics for beam A. The coupling resulted in false early/late decisions for beam A. Similarly, the SB from A also created a false early/late decision for beam B.
Thus, a need has long existed for a method to minimize CCI in uplink beams of a communications satellite system to minimize false reception of a synchronization burst.
One object of the present invention is to reduce co-channel interference (CCI). Another object is to allow highly reliable maintenance of time synchronization between a satellite and a terminal. Another object is to allow highly reliable maintenance of time synchronization between a satellite and a terminal in a Time-Division Multiple Access (TDMA) satellite communication system.
Another objective of the present invention is to minimize the cost and complexity of hardware used to mitigate the co-channel interference in synchronization in a satellite communication system.
One or more of the foregoing objects are met in whole or in part by a method for mitigating Co-Channel Interference (CCI) between synchronization bursts (SB) in a multi-beam communication system. The present invention uses different pseudorandom noise (PN) sequences among each of the various same-color beams. In particular, different phases of the same maximal length (ML) sequence may be used in each same color beam. Because two SBs based on differing phases of an ML sequence have a small cross correlation, SB CCI between the same color beams is minimized at the receiver. A correlation boost is provided in the processing electronics for beam A when the electronics receive the SB for beam A. However, due to the low cross correlation between SBs, the processing electronics for beam A do not provide a correlation boost to CCI generated by beam B. Timing errors from beam B are therefore not coupled into the measurements made for beam A. As a result, the timing error derived for beam A is much more reliable.
These and other features of the present invention are discussed or apparent in the following detailed description of the preferred embodiments of the invention.